The present invention is directed to methods and apparatus to non-destructively read and play the audio signal of a standard vinyl phonograph record. Specifically, the present invention relates to audio systems which employ light beams and optics, rather than a mechanical stylus, to follow the spatial modulations of the record groove.
Conventional analog audio records are made of vinyl records which store information in the grooves. The grooves are typically formed as left and right walls cut into a master disk with a mechanically vibrating stylus. The physical geometry of the left and right groove walls, known as modulations, carry the audio waveform as recorded information. Many conventional vinyl records are reproduced from the master disk.
A record is played with a device commonly known as a "pick-up." Typically, a stylus or needle is mounted in the pickup, and lowered into the groove, which acts as a guide for the stylus to follow. The stylus is in physical contact with the groove. As the record is rotated on a turntable, the changes in the right and left groove wall geometries, that is, the surface displacements, cause the stylus to vibrate. The stylus vibrations are then converted into electrical signals that are delivered to the input of an amplifier.
Conventional analog recording and reproduction systems have several inherent flaws. Imperfections in the recording medium and in the reproduction system can alter the playback waveform and distort the sound. Conventional reproduction systems are sensitive to "ticks" and "pops" caused by imperfections or dust particles in the grooves, surface scratches and low frequency noise caused by the system's physical vibration. In addition, since the stylus is in physical contact with the record groove, record wear (typically in the form of pits in the groove walls) is inevitable. Lastly, since conventional reproduction systems are mechanical, inertia affects the response characteristics.
In recent years, there has been research into and development of optical recording and reproduction systems. In general, digital reproduction systems use laser sources (coherent light) to detect the presence or absence of reflected light from recorded "pits" along a track. These digital systems require recording, processing and reproduction techniques very different from those for the conventional analog record. The use of light and optics to play conventional analog records has largely been ignored or found to be unsatisfactory.
U.S. Pat. No. 4,363,118 (Roach et al) entitled "Sound Carrier Amplitude Measurement System" describes the use of optical detection systems to provide an estimation of signal depression depth of spiral grooves for video disk records. Video disks are different from conventional analog recording disks as shown by the following comparisons:
______________________________________ Conventional Analog Video Disk Record ______________________________________ 9500 grooves/inch 250 grooves/inch 140.degree. V groove 90.degree. V groove 0.5 microns depth 50 microns depth FM encoded AM encoded ______________________________________
In the Roach patent, a diffraction pattern is used to view a plurality of grooves in order to determine nominal groove depth of the illuminated region, rather than to extract encoded data, as is the purpose of the present invention. The diffraction pattern of Roach does not provide extractable data about the individual grooves observed and therefore cannot be used to reproduce sound.
U.S. Pat. No. 3,327,584 (Kissinger) entitled "Fiber Optic Proximity Probe" describes a fiber optic proximity probe used to measure very minute shaft rotation, vibration or displacement, stress or strain, surface testing, and rotation counting but not for extraction of audio data.
Reissued U.S. Pat. No. 30,723 (original U.S. Pat. No. 3,992,593) to William K. Heine entitled a "Disc Phonograph Record Playback by Laser Generated Diffraction Pattern" plays conventional disc phonograph records using a laser and means for detecting the resulting interference-diffraction pattern of arcs of light. The Heine patent is limited to the analysis of interference-diffraction patterns and is not believed to provide an entirely satisfactory system.
While the use of optics in connection with phonograph records and video disks has been proposed in one form or another, a satisfactory and economical optical system for the optical playback of conventional phonograph records is still not available.
In light of the above background, there is a need for an improved, economical optical turntable system capable of playing conventional phonograph records.